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OMC Cobra 4.3 burned valve or what?

S

Scrid

Member
Hi guys, Hey I recently changed plugs on my 1993 OMC Cobra 4.3 from NGK plugs to some AC delco platinum plugs ( 36 bucks for the set ) my brother recommended.
Well I put about 5 hours on the new plugs and noticed a miss not long after installing them.
I wasn't impressed with the new plugs anyway so I decided to change back to NGK.
When I pulled the plugs I found the #6 plug missing the electrode and the 90 degree tip was melted back about halfway.
Thought Great!... found my miss!
Put the new NGK plugs in and still have a miss! Checked compression and I only have 25lbs on the #6 cylinder.
I saved my old NGK plugs and looked at them...they all look ok.
Now I'm wondering if the electrode on the AC delco plug got stuck under a valve face?
I'm about the run that cylinder ) #6) up to TDC... blow some compressed ait in the plug hole and find out if it the intake or exhaust that's causing the problem.
Any other thought?
Thanks.
Chris.
 
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Were the platinum plugs the same length?
Your gonna need to get a bore scope or pull the head.
Sounds like your gonna need to pull the head anyway?
Take pics of all steps.
Maybe there was a defective plug? Not sure AC/Delco will do anything.
 
Pull the valve cover remove the rocker arms on that cyl and check the valve height. A tuliped valve will be higher. Watch out for the crank movement when pumping in the air.
 
Yes that was my next step today.
I hope I can get the valve cover off without taking the exhaust manifold off but like jerryjerry05 said I may be pulling the head anyway.
I was hoping maybe a carbon buildup on the valve stem not allowing it to close.
 
I like the Bore Scope idea. Nothing to loose by giving it a shot.

Agree.... watch the air pressure while the piston is near TDC C/S.
It may not take much pressure to move the piston downward..... although unless your clothing or fingers were to get caught in XYZ, there's really not much risk.


Also, if you're going to pull the one cylinder head, why not pull both?
If you clearn up the Starboard bank cylinder head valves only, your boat will make more dramatic Port side turns. :D :D

Seriously...... do both banks while you're in this deep!


.
 
Ha thats funny!
Yeah not sure I can get my hands on a borescope.
Can I just use automotive engine gaskets on this unit?
I have a car parts store less than 3 blocks away.
If I find a damaged valve I may just go ahead and install two rebuild heads.
 
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Scrid said:
Yeah not sure I can get my hands on a borescope.
See if you can rent one.

Can I just use automotive engine gaskets on this unit?
As in "Head Gaskets"?
If Raw Water cooled, NO!
If equipped with Closed Cooling, yes.

If I find a damaged valve I may just go ahead and install two rebuild heads.
Make sure that you check the casting numbers so that you go back together with the same size combustion chambers.
Click to expand...
 
OK, so I pulled the exhaust manifold, and valve cover. Removed all the rockers and found the #6 cylinder intake valve is up about an 1/8" higher then the rest.
Blowing compressed air in the cylinder and the valve still seals.
Looks like a tuliped valve.
Now in your guys opinion what would have caused this a stuck lifter that wasn't allowing the valve to close therefore letting the intake valve get to hot and bend?
 
Looks like I had a headgasket starting to fail between cylinders 3 and 5, and on other head 2 and 4.
Need one new valve for the #6 cylinder.
 
Ayuh,.... Sounds like yer Skippin' might be Detonation, from the wrong spark plugs, or more likely a Lean condition,...

Thoroughly check out yer Fuel system,...
 
Just curious, did you find why the #6 I valve is "higher"? Has the valve seat sunk or worn that much? I would think a sticking intake valve would "pop" out the carb noticeably.
 
The # 6 valve was tuliped so thats why it was higher.
Was gonna lap in a new valve and replace the lifter but in the mean time I found that as my luck goes both cylinder heads are cracked on each rear cylinder between the intake and exhaust valves.
Poop!
 
Scrid said:
The # 6 valve was tuliped so thats why it was higher.
Was gonna lap in a new valve and replace the lifter but in the mean time I found that as my luck goes both cylinder heads are cracked on each rear cylinder between the intake and exhaust valves.
Poop!
Click to expand...
Get a rebuilt long block and call it a day. Tuliped valve may indicate a lean condition. IE crack as you have found.
 
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